Spinal fusion is utilized to correct painful movement across degenerated or unstable spinal segment and is supplemented by fixation with screws. Screws are commonly placed through the posterior vertebral arch. Multiple fixation targets have been used in the past including facets, spinal processes, lamina, transverse processes, lateral masses and pedicles. Currently fixation through the pedicles offers the most robust purchase and is utilized when possible.
Screws are implanted from the back to the front of the vertebra with one screw inserted per each side per vertebra. Individual screws have to be rigidly linked to eliminate movement between corresponding vertebras. (U.S. Pat. No. 5,474,555 Puno et al.). Anatomy of individual patient makes it difficult and often impossible to line up pedicle screws in one plane. Screws project with variable medial-lateral displacements and different angulations. This places significant challenges on the mechanical design of the linking system. These complex mechanisms with multiple joints have to be manipulated in a deep wound restricted by muscle and soft tissue.
Most of the available systems can be broken into one of three groups: plating system (U.S. Pat. No. 4,611,581); top loading rod system (U.S. Pat. Nos. 4,805,602, 5,360,431) and side loading rod system (U.S. Pat. No. 5,261,909).
The top loading rod systems utilizes a seat for the rod that projects along the screw trajectory and forms a ball and socket joint with the spherical screw head. It offers several degrees of freedom about the screw axis: (i) 30-40 degrees of angular deflection from the screw axis (ii) 360 degrees of rotation about the screw axis. Any further adjustments have to be gained by contouring the rod to fit into the rod receptacle of the seat. These systems offer advantages when the exposure is directly over the screw such as in muscle splitting or minimally invasive approaches.
In a standard midline approach screws project out into the soft tissue at the lateral extreme of the exposure. Top loaded systems require manipulation at this lateral extreme and side loading rod system can offer advantages in these circumstances. In side loading rod systems connectors run perpendicular to the screw trajectory and the rod is placed on the side of the screw toward the center of exposure U.S. Pat. No. 5,261,909).